1. Field of the Invention
My invention pertains to the art of electroplating with the use of anodes of the nonconsumable type, commonly known as insoluble anodes, and in particular to a method of and apparatus for replenishing a plating solution with the metal to be deposited in such electroplating operation. The method and apparatus according to our invention have particular utility in conjunction with continuous electroplating or electroforming apparatus of the type disclosed in Yamashita et al. U.S. Pat. No. 4,053,370 dated Oct. 11, 1977, of the type described and claimed in Yamaguchi U.S. Pat. No. 4,119,516 dated Oct. 10, 1978, and of the type described and claimed in the above referenced application entitled "Apparatus for Electroplating Strip Material without Current Leakage".
2. Description of the Prior Art
The above cited U.S. Pat. No. 4,119,516, for example, proposes electroplating apparatus useful for the fabrication of metal foil, for use in the manufacture of printed electronic circuitry, or printed circuit patterns on a continuous strip of electrically conductive material such as stainless steel or nickel. The apparatus comprises a cathode under which the continuous strip is fed slidingly and thereby made cathodic, and two insoluble anodes underlying the cathode, with an interelectrode gap between each anode and the conductive strip traveling under the cathode.
An electroplating solution for use in the prior patented apparatus contains the metal to be deposited, notably copper. Made to flow turbulently through the interelectrode gaps, the solution speedily and uniformly deposits the metal on the downward-facing surface of the conductive strip advancing continuously over the successive anodes. The surface of the conductive strip must previously be masked with a plating resist for the fabrication of circuit patterns and can be blank for foil production.
With the progress of the electroplating or electroforming operation the bath becomes gradually depleted of its metal content. A periodic replenishment of the bath is therefore a necessity to keep the metal concentration therein within required limits. Toward this end the plating industry has had to purchase a soluble compound of the metal, for introducing its solution into the bath. In the case of an acid copper plating bath, for example, copper has been added in the form of a solution of purchased copper sulfate. This practice is objectionable for the following reasons:
1. The added copper sulfate solution inordinately increases the volume of the bath if the electroplating apparatus incorporates an electrolyte recovery system such as that proposed by the above noted application entitled "Semiclosed Electrolyte Recovery System for Electroplating Apparatus". The bath volume increases even more in the case where insoluble anodes are used.
2. With the use of insoluble anodes, sulfate ions increase, lessening the solubility of copper sulfate and causing its recrystallization and consequent settling down to the bottom of the plating tank.
3. The copper sulfate solution can introduce such cation contaminants as iron, nickel, zinc, and chromium, increasing the internal stress of the electrodeposited copper, decreasing its elongation ability and malleability, and impairing its hardness and other properties.
4. Copper sulfate is not so cheap as can be desired.
Ultimately, therefore, the plating solution must be discarded. Thus the conventional method of bath replenishment is not, or at least hardly, compatible with the electrolyte recovery system.